Method for removing defects from slabs and blooms of steel and other metals

ABSTRACT

One or more electric arcs take place from fluid-cooled arcing surfaces at fixed positions along the path of movement of a slab which is to have defects removed therefrom, and magnetic fields generated in the electrodes cause the arcs to move substantially continuously in repetitive paths over the arcing surfaces and over the surfaces of the slab. The repetition rate of movement of the arcs is sufficiently large and the rate of movement of the slab is sufficiently small whereby the arc spot occurs at substantially every point on the slab surface. Additionally, means is provided for quickly cooling successive portions of the surface of the slab after said portions have been heated by the electric arc or arcs. In some embodiments all surfaces of the slab are heated during linear movement of the slab in one direction; in another embodiment, two surfaces of a slab generally rectangular in cross section are heated while the slab moves in one direction, the slab is thereafter turned over and moved back in the opposite direction during which later movement the other two surfaces of the slab are heated.

1"] (medals own-=71 x1e 3959mm? [72] Inventors Daniel A. Moniero3,146,336 8/1964 Whitacre 1 219/121 Plttlinrgh; 3,211,886 10/1965 Barkanet al... 219/121 George A. Kelaely. Export, Pm; Armin M. 3,336,4608/1967 Hauck et al.." 219/121 l, um. Wis. 2,472,851 6/ 1949 Landis etal. 219/ 123 1211 Appl. No. 801,502 3,102,946 9/1963 Fonberg 219/1233,248,514 4/1966 Ramsey 219/123 atente a y 1 [73] Assignee WestinghouseElectric Corporation i Assistant Examiner-J G. Smith A ATSttt CLMHIdMlHll Division ofSer. N0. 4394832. Mar. 15. 1965. a e abandoned,continuation of application Ser. No. 817,283. Apr. 15, 1969.

ABSTRACT: One or more electric arcs take place from lluid- Y cooledarcing surfaces at fixed positions along the path of movement of a slabwhich is to have defects removed 5 METHOD FOR REMOVING DEFECTS FROMtherefrom, and magnetic fields generated in the electrodes m AND ooms OFSTEEL AND OTHER cause the arcs to move substantially continuously inrepetitive METALS paths over the arcing surfaces and over the surfacesof the 1 Chime Davies n slab. The repetition rate of movement of thearcs is sufficiently large and the rate of movement of the slab issufficiently small US.

whereby the are spot ocpurs at subsantially every point on the 219/ slabsurface. Additionally, means is provided for quickly coolllt. inssuccessive portions of the surface of the slab afie por- 1 Field ofSearch 219/69, one have been heated by the electric ate or area In someP1 bodiments all surfaces of the slab are heated during linear movementof the slab in one direction; in another embodi- [s6] Rdenm cued mcnt,two surfaces of a slab generally rectangular in cross sec- UNITED STATESPATENTS tion are heated while the slab moves in one direction, the slab2,125,172 7/ 1938 Kinzel 219/121 is thereafter turned over and movedback in the opposite 2,993,983 7/1961 Carpenter et a1. 219/124 directionduring which later movement the other two surfaces 3,050,616 8/1962 Gage219/121 of the slab are heated.

PATENTEDJULETIS?! 3596047 SHEET 1 OF 5 9 (9 35 II LL. :3 6 N min M.uning ATT NEY PATENTFUJULZYISH 3,596,047

sum u I]? 5 MELTING POINT MELT ZONE m CENTER LINE 0 OF SLAB w w 5 F2500" 0: U1 0. 2 LL] INITIAL TEMP. 240...WH. .m-

4 e 0 0.5 l 2 a DEPTH BELOW SLAB SURFACE-INCHES METHOD FOR REMOVINGDEFECTS FROM SLABS AND BLOOMS OF STEEL AND OTHER METALS This applicationis a division of copending application Ser. No. 439,832 filed Mar. l5,I965, now abandoned, the invention of said application being describedand claimed in a copending continuation application Ser. No. 8l7,283,filed Apr. 15, 1969.

This invention relates to improvements in a method of and apparatus forremoving defects from slabs and blooms of metals such as steel, and moreparticularly to new and improved apparatus for heating the entiresurface of a slab to a predetermined depth by electric arcs taking placeto substan ,tially 100 percent of the slab surface as the slab is movedsubstantially continuously in relation to the arcs, to thereby treat theentire surface of the slab and remove defects therefrom.

The method generally employed at present in steel mills to eliminatedefects in a slab is a method known as hot scarfing.

The hot scarfing method is used not only with slabs but with blooms ofno particular shape. In the hot scarfing method, the flame from amultiple head torch is brought into contact with all of the surfaces ofthe slab. The torch uses a fuel gas and ex- .ygen mixture; the surfacetemperature of the slab is raised to the point where rapid oxidation ofthe hot surface occurs in the presence of the oxygen stream. The slab isthen moved past the torch and the heat generated by the exothermicoxidation reaction is sufficient to sustain the process without furtheruse of the fuel gas. In the process as much as 0.05 inch of metal isremoved from all surfaces of the slab with the oxides of iron beingblown oh the slab surfaces by the high gas exit velocities of the torchheads.

The hot-scarfing method is characterized by several disadvantages. Oneis that subsurface defects which are not removed during the scarfingmust be later identified and burned out by manually operated scarfingtorches prior to further working. Another is the considerable loss ofmetal which results from the scarfing operation. The metal removed up to0.05 inch thick may correspond to a loss in mill output of as much as Mspercent. In addition to the actual value of the metal lost, there areadditional costs of carrying away the dross from the scarfing operationand returning it to the furnaces for remelting.

An additional disadvantage of the hot-scarfing method is that largequantities of oxygen are required in scarfing to burn away the surface.An oxygen-generating plane must be operated to supply this gas. Theoxygen capacity of the equipment, production rate of the blooming mill,and the economic balance of metal loss and handling combine to limit themaximum thickness of metal which can be removed by hot scarfing.

Furthermore, slabs have surface or subsurface defects, as mentionedhereinbefore, which extend beyond the metal layer removed, which must beidentified, and the slabs must be moved to an area where manual scarfingtorches are used to remove the defects. The special handling of slabswith defects results in separation of slabs produced for a particularorder and requires detailed record keeping and identification to assurethat the slabs are included in the correct shipments.

Apparatus for practicing the method of the present invention makes useof arcs from a number of round or elliptical arc electrode headsbelonging to nonconsumable water-cooled electrodes, the electrodescontaining internal stirring coils for producing magnetic fields tosubstantially continuously move the arcs around the arcing surfaces ofthe electrodes. An electric arc takes place simultaneously between eachelectrode and the surface of the slab. The magnetic field produced bythe coil rotates the are on the electrode face and the arc is draggedalong the slab surface. Heat generated in the slab by radiation from theare, by resistance heating at the point of contact of the arc, and heatof convection of hot gases in the vicinity of the are, results inmelting the slab to a depth determined by the material of the slab, byare current and speed of rotation, and sla'b temperature and speed ofmovement.

Cracks and other defects in the slab surface are healed as the metal ismelted by the arc and then cooled by spray from fluid manifolds. Solidand gaseous impurities are floated to the slab surface in the moltenzone where they either escape to atmosphere or else are retained for alater removal. In one embodiment of apparatus for practicing theinvention, during the surface heating and cooling oxidation of thesurface is prevented by providing a reducing gas or inert gasatmosphere.

In further summary in one embodiment of apparatus for practicing themethod of the invention a plurality of electrodes are disposed adjacentthe upper surface of the slab, an additional plurality of electrodes aredisposed adjacent the lower surface of the slab, with smaller numbers ofelectrodes disposed on each side of the slab so that the entire slabsurface is heated as the slab moves along. Disposed near the arcelectrodes are fluid spray devices for quickly cooling portions of thesurface after movement of the slab has moved any heated portion awayfrom the vicinity of the arc electrodes. In another embodiment,electrodes on the top and one side of the slab heat these surfaces asthe slab moves in one direction, whereafter the slab is moved to adevice for turning the slab over, and the slab is thereafter moved pastthe electrodes in the opposite direction whereupon the former bottomsurface and the other side surface are heated, spray cooling devicesbeing disposed on both sides of the electrode assembly.

Accordingly, one object of the invention is to provide a new andimproved method for removing defects from slabs and blooms of steel andother metals.

Another object is to provide new and improved apparatus for removingdefects from slabs and blooms of steel and other metals.

A further object is to provide new and improved apparatus employingnonconsumable arc electrodes for remelting the surface of a slab orbloom of steel or other metal to a predetermined depth to eliminatedefects therefrom, and employing means to quickly cool the remeltedsurface to avoid oxidation thereof.

Still a further object is to provide new and improved bloom and slabremelt arc heater apparatus.

An additional object is to provide new and improved apparatus forheating the surface of a slab or bloom to a predetermined depth.

These and other objects will become more clearly apparent after a studyof the following specification, when read in connection with theaccompanying drawings, in which:

FIG. I is a view in perspective of apparatus for treating the top,bottom and both sides of the slab in a single movement and utilizingfour large electrodes which may be generally cylindrical for heatingeach of the four surfaces of the slab;

FIG. 2 is a view of a second and preferred embodiment of apparatus, inwhich a number of electrodes spaced in predetermined patterns are usedfor each surface of a wide slab during the slab remelt operation;

FIGS. 3A and 3B are views of the electrode arrangement of FIG. 2; i

FIG. 4 is an electrical circuit diagram of a simplified version of theapparatus of FIG. 2, in which two electrodes with overlapping patternsof arc movement are used for the top of the slab, two similar electrodeswith overlapping patterns of arc movement are used for the bottom of theslab, and one electrode is used for each side of the slab, all of theseelectrodes being fed from one three-phase high voltage transformersource;

FIGS. 5A and 5B are schematic electrical circuit diagrams illustratingthe effect of an inductor or lack thereof in series with each pair ofcooperating arc electrodes;

FIG. 6 is a perspective view of apparatus according to a fourthembodiment of the invention in which the steel slab is turned over tocomplete the process of heating all surfaces of the slab;

FIG. 7 is a graph illustrating the operation of the apparatus ofFIGS. l,2 and 6; and

FIG. 8 is an elevational view partially in section of a nonconsumableelectrode. 7

Referring now to the drawings for a more detailed understanding of theinvention, in which like reference numerals are used throughout todesignate like parts, and in particular to FIG. 1 thereof, a slab to beheat treated is generally designated 10, is mounted upon rollers ll, l2,l3, l4 and on the one hand, and 16, 17, and 18 on the other, havingdisposed therebetween two are electrode assemblies or C- framesgenerally designated 21 and 22, the electrode assembly 21 being arrangedfor the purpose of heating the top 23 and bottom of the slab, and theelectrode assembly 22 being constructed and arranged to heat the twosides of the slab, one of these sides being shown at 24. in accordancewith conventional practice, motors may be disposed at spaced intervalsalong the path of travel of the slab 10 to apply turning forces tocertain of the rollers, for example motor 19 and roller 18, certainother rollers or groups of rollers intermediate the driven rollers beingjournaled for free rotation while supporting the slab in its translatorymovement.

The electrode assembly generally designated 21 is seen to include anupper housing, shroud or muffler 28 and a lower housing or shroud 29.Disposed within the upper shroud 28 there is seen an upper nonconsumableelectrode 31, It will be understood that the lower housing or shroud 29has a similar electrode, not shown, disposed therein. The electrode 31is of the nonconsumable type, that is, it includes a water-cooledelectrode face member with a coil disposed adjacent thereto or withinthe electrode, the coil being constructed and arranged to set up amagnetic field of sufficient magnitude transverse to the current path ofthe are between the electrode and the slab to provide that the arc movessubstantially continuously around the electrode face member at apredetermined speed, in accordance with the current through the arc andthe construction including the dimensions of the electrode, so thatsubstantially no sublimination of electrode material from the electrodeface member occurs, and no burn through as a result of the intensely hotare spot occurs.

The factors which must be considered in manufacturing and constructing anonconsumable electrode which will withstand the most severe operatingconditions, including very high current conditions of a substantiallycontinual nature, are reheating apparatus in the instant case, are setforth fully in the copending application of A. M. Bruning for "ElectricArc Furnace and Nonconsumable Electrode For Use-Therein," Ser. No.407,332, filed Oct. 29, 1964, and assigned to the assignee of theinstant invention,

Disposed within the cylindrical electrode 31 and spaced therefrom is apipe 34 for bringing an insert gas and exhausting the gas on the heatedslab in the area of the arc. The gas brought in by pipe 34 may also bechosen to establish a reducing atmosphere at the heated slab. The slab10 moves generally from left to right in the figure and accordinglythere is provided a spray tube water manifold, or spray pipe 36 forspraying cold water or other fluid on successive areas of thehot surfaceof the slab substantially immediately after these areas of the surfaceof the slab move out from under the shroud. It will be understood thatthe pipe 36 has a plurality of spaced apertures along the length thereofso that cooling fluid may be substantially directly applied to allsurface portions of the top 23 of the slab 10 along any given lateralpath.

it will be understood that means symbolized by lead 26 is provided forbringing electrical current to the electrode 31; means is provided forconnecting the terminal of opposite polarity of the source of the slab,symbolized by lead 27. In actual practice, certain rollers of conductivematerial may conduct current to the slab, the rollers having spacedteeth scattered over the surface thereof to bite into the slab and makegood electrical contact. Also, spring biased roller electrodes on theside of the slab can be employed.

Means is also provided for bringing a cooling fluid to the electrode 31and conducting fluid therefrom after the fluid has passed around theelectrode face member and possibly other parts of the electrode toconduct heat therefrom. The tubular electrode supporting structure 32may itself be at least partially conductive to conduct electric currentto a conductive electrode face member.

Within the housing, shroud or muffler 29 beneath the slab 10, it will beunderstood that there is disposed an energized electric arc electrodewhich may be substantially similar to electrode 31, and which may have apipe corresponding to the pipe 34 for bringing an inert or reducing gasto the area of the :slab surface the arc impinges on. The lower shroud29 has the fluid pipe or manifold 38 disposed in a manner to spray acooling fluid on the bottom of the slab as the slab passes by.

Spaced a predetermined distance from the electrode assembly 21,depending upon the speed of the travel of the slab 10 and other factorssuch as the temperature of the slab, the materialthereof, and the depthto which the slab is to be heated, is the second aforementionedelectrode assembly 22 mounted on a C-frame and having shrouds 41 and 42each containing an electrode, not shown, having a diameter at least asgreat as the height of a side of the slab, which it is un-' derstood aresuitably connected to a source of potential to produce arcs to the slab10; the electrodes of shrouds 41 and 42 also contain a pipe or pipes,not shown, for bringing an inert or reducing gas to the area of the arcand releasing it. Disposed adjacent the exit side of the shroud 41 is aspray pipe or manifold 43; disposed adjacent the exit side of shroud 42is a spray pipe or manifold 44, for spraying the two sides of the slabto quickly cool successive areas of the same after the areas have passedthrough the heating areas of the arc electrodes in shrouds 41 and 42.

[n the operation of the apparatus of FIG. 1, as previously stated anelectric arc is struck between each electrode and the surface of theslab. In FIG. l the slab is assumed to be of a relatively small widthand a relatively small height. For example, the dimensions of a bloom orslab frequently encountered in practice is 7% inches by 8% inches.Assuming for the purpose of discussion that the top of the slab is 8%inches in width, then the diameter of the arcing surface of theelectrode 31 would be made approximately 8% inches in width, or slightlyover, so that the are traveling around the arcing surface in response tothe aforementioned magnetic field continuously moves back and forthacross the entire upper surface of the slab. it will be apparent thateven though the slab may be moving a speed of several inches per second,the arc of the electrode 31 may be made to rotate at a speed of, forexample, 3,000 revolutions per second, this being a speed which iseasily obtained in practice, so that assuming that the slab moves 3inches per second and the arc moves 3,000 revolutions per second it willbe seen that the arc will traverse the slab approximately 2,000 timesper inch, or will make approximately l,Q00 return trips across the slabper inch, so that for all practical purposes every spot on the slabcomes in direct contact with the are from electrode 31. The electrode inshroud 29 has the same shape, in other words, has a diameter at least asgreat as the width of the bottom of the slab, and the arc therefrom canrotate at the same speed.

The electrodes in shrouds 41 and 42 have diameters substantiallycorresponding to the height of the sides of the slab, or slightlygreater, and the arcs therefrom can rotate at the same speed inaccordance with the current in the arc and the strength in the magneticfield. Formulas for calculating the speed of the arc from the fieldstrength and the arc current are set forth in the aforementionedcopending patent application of A. M. Bruning. It will be readily seenthatother factors may enter into the desired speed of travel of theslab, and the desired speed of rotation of the arc. For example, thedepth to which the slab is to be heated is a factor; if it is desired toheat the slab to a greater depth, then the speed of movement of the slabcan be slowed, the speed of rotation of the arcs can be increased, orthe current in the arcs can be increased.

Formulas for relating the speed of the movement of the arc, speed ofmovement of the slab, arc current, and material, to the required heatflux for melting the slab to a predetermined depth will be set forthmore fully hereinafter.

leads 51, 52, 53, 54, 55 and 56 respectively, and having dimensionsapproximately as shown. A lower shroud 70, it will be understood, hasthe same number of electrodes of similar dimensions spaced in the samemanner positioned therein, with means not shown for convenience ofillustration, for bringing electrical current to the electrodes toproduce arcs from the electrodes to the bottom of slab 50. Pipes 79 and80 bring an inert gas to the shroud 69, the gas being exhausted or freedin the area adjacent the arcing surface of the slab. The

shrouds 69 and 70 have fluid spray pipes or manifolds 81 and 82respectively on the exit sides thereof for spraying the upper and lowersurfaces of the slab with a cooling fluid. It will be understood thatpipes, not shown for convenience of illustration, also bring an inertgas to the electrodes of the lower shroud 70, this gas also being freednear the arcing surface of the slab.

Rollers 161-167 move the slab; roller 167 is shown as being driven bymotor 168. Roller 161 may also be driven by a motor, not shown forconvenience of illustration.

It will be understood that if desired additional rollers, not shown forconvenience of illustration, may be disposed between electrode assembly61 and electrode assembly 62.

portion of the side of the slab. In a like manner the aforementionedelectrodes 71 to 76 are as aforementioned so placed -with overlappingpaths of arc movement that the top 66 of the slab has every spot thereontraversed by an arc, depending upon aforementioned factors of speed ofrotation of the arcs, and the speed of movement of the slab. In likemanner the bottom 67 of the slab has every point thereon subject tocontact with the arcs from one of the aforementioned electrodes 91 to96.

The apparatus of FIG. 2, by providing six arcs to cover or care for theentire upper and lower surfaces-of the slab 50, permits a proportionallylarger or more frequent contact of the arc with any given area of theslab, and more heat can be obtained in this fashion for treating a widerslab, or treating a given slab to a greater depth, or melting thesurface of the slab in a shorter time.

Particular reference is made now to FIG. 4 wherein an electrical circuitaccording to a third embodiment of the invention is shown. In theapparatus of FIG. 4 two electrodes 101 and 102 produce arcs 103 and 104respectively to the top surface of a slab generally designated 100. Atthe sides of the slab 100 one electrode has a sufficiently largediameter to cover an entire side, one of these side electrodes beingshown at 105, producing an are 106, the other electrode being shown at107 producing an are 108. At the bottom of the slab electrodes 109 and110 produce arcs 111 and 112 respectively.

In FIG. 4 the six electrodes 101', 102,105, 107, 109 and 110 areconnected to a three phase source of supply. A three phase transformerhas its primaries 117, 118 and 119 Y-connected with the junction betweenprimaries connected to ground 120,

' and leads 114, 115, 116 connected to a suitable source of It is seenin FIG. 2 that any portion of the slab 50 after moving past the areaadjacent the fluid cooling pipes 81 and 82 1 moves adjacent theaforementioned second electrode as-,

sembly 62, which has shrouds 85 and 86 disposed adjacent the v sides ofthe slab 50, each of the shrouds containing a pair of electrodes, theelectrodes of shroud 86 being shown at 77 and 78 with leads 57 and 58respectively. Disposed adjacent the exit sides of the shrouds 85 and 86are a pair of fluid spray pipes or manifolds 89 and 90 for spraying thesides of the slab with a cooling fluid after any given portion of theslab has past through the heating area of the shrouds. It will beunderstood that the shroud 85 has a pair of electrodes, not shown forconvenience of illustration, similar to electrodes 77 and-78, and

I shroud 85 has means, not shown, for bringing potentials to theelectrodes therein to form arcs to the side surface of .the slab 50. Itwill further be understoodthat both shrouds 85 and 86 have pipes orother means, not shown for convenience of illustration, for bringing aninert gas into the shroud and releasing the gas at the area or areaswhere the arcs take place to the surface of the slab or bloom.

Particular reference is made now to FIG. 3A, which is a composite viewin which the upper electrodes of assembly 61 are shown in cross section,and the electrodes of both shrouds 85 and 86 of the second electrodeassembly 62 are shown three phase alternating current potential.Secondaries 121, 122 and 123 are also Y-connected with. the junctiongrounded at 120, and it is seen that the slab 100 is also electricallyconnected to ground120. Secondary 121 is connected by way of lead 125and inductor 126 to the aforementioned electrode 105, and lead 125 isalso connected by way of inductor 127 to secondary 122is connected bylead 128 and inductor 129 to electrode 107, and lead 128 is alsoconnected by inductor 130 to electrode 110. The aforementioned secondary123 is connected by lead 131 and inductor 132 to electrode 101, and thelead 131 is also connected by way of inductor 133 to electrode '102completing the electrical circuit for forming the six arcs to the slab100.

Particular reference is made now to FIGS. 5A and 5B. In FIG. 5A twopairs of electrodes A and B are shown connected across a singlesecondary, by way of a choke or inductor D whichcarries the current ofboth arcs. Let it be assumed for purposes of illustration that secondaryC supplies 2,000 volts and that it requires a voltage of 1,000 volts tostart an are at electrodes, B. Assume further that an arcis taking placeat the pair of electrodes A. The voltage drop across the electrodes A isthe same as that across electrodes B and is so small that the necessaryvoltage will neverbe built up across the electrode B generally in plan.The purpose of the composite view of FIG. I

3A is to more clearly illustrate that the six electrodes 71 to 76 are sodisposed that any point on the top surface of the slab passes throughthe arc of at least one of the six electrodes as the slab 50 moves withrespect to the electrode assembly and the arcs rotate. The electrodes ofthe assembly 62 are seen as spaced along the path of movement, theelectrodes of shroud 86 being shown at 77 and 78, and in FIG. 3Aelectrodes 83 and 84 of shroud 85 are also shown.

In FIG. 38, to which particular attention is directed, the slab 50 isshown in relationship to all of the electrodes of both electrodeassemblies 61 and 62. In FIG. 3B, the. aforementioned electrodes on thebottom shroud 70 are shown at 91 to 96 inclusive. From FIG. 38, it isseen that the side electrodes 77 and 78 overlap each other in a verticaldirection, so that no' are so placed that no portion of the side 65 ofthe slabcan pass the electrodes without having the arc traverse the areaof that to, cause an arc to start. Assume now that circuit is connectedas shown in FIG. 5B, in which electrodes A and B have individualinductors or choke coils E and F connecting them to the secondary C. Ifthe arc starts at A before it does at B, even though a voltage, forexample, only 500 volts exists across the arc A, an additional voltagedrop of 1,500 volts may exist across the series-connected choke. This ison the assumption that secondary C supplies a voltage of 2,000 volts.Since no current is flowing in the choke or inductor F,.no voltage dropoccurs there across, and the entire 2,000 volts is developed across thepair of electrodes B sufficient to start the arc. Such .an arrangementof .an individual inductor in series witheach The secondaries could alsobe Delta connected, without requiring any electrical connection to theslab.

Particular reference is made now to FIG. 7, where the graphs illustratethe relationship between the average temperature of the slab or bloom,and the depth to which the slab is heated beneath the surface for anumber different dwell times" or heating times or passage times, itbeing assumed that the power in the arcs remains substantially constant.

The family of curves of FIG. 7 shows the difierent temperatures to whichthe slab is heated at any given depth as the time employed to melt thesurface of the slab to a depth of 0.1 inch is varied. Assuming for thepurposes of discussion an initial slab temperature of 2,400 F, and thatthe slab melts at 2,700 F, if the time used to melt the surface of theslab to a depth of 0.1 inch is one second, it is seen from the curvesthat a point in the sla 0.5 inch from the surface or 0.5 inch deep isheated to 2,410, whereas in three seconds the same point 0.5 inch deepis heated to 2,460".

The temperature of the point, and those of other points of other depths,can also be obtained for other heating times from the curves.

Since the longer the time required to melt to 0.1 inch, for any givendepth in the slab, the greater the temperature the slab at that depth isheated to, it is apparent that additional heat energy is required at theslower speed. From the curves of FIG. 7 it is seen that the integratedarea under the three second curve is greater than the integrated areaunder the one second curve. This integrated area represents heat used inheating the slab in addition to the heat required to melt the surface toa depth of 0.1 inch. Since heat energy used in heating the core of theslab serves no useful purpose, it is seen that the longer time of 3seconds represents decreased efficiency.

From the one-second curve, it is seen that after a period of one secondthe temperature distribution in the remainder of the slab is such thatat a depth of approximately 0.025 inch from the surface, the metaltemperature has risen to 2,500 F. The curves of FIG. 7 are useful incalculating the additional heat required to balance loss to the core ofthe slab in the following somewhat simplified calculations of anexemplary condition which might be met in practice, where a slab size of6X l is assumed, an initial slab temperature of 2,400 F is assumed, andit is assumed that the material of the slab melts at 2,700" F.:

Specific heat required to-raise the metal temperature Assume melting toa depth of 0.1 inch: (and a uietnl The additional heat required tobalance loss to core of slab, where q" is the energy per unit area,obtained from the curve of FIG. 7 by integrating the area under thecurve:

B.t.u. ft. B.t.u. T X ike. f.

From power equivalent tables such as those found in a handbook forelectrical engineers, it is found that one B.T.U. is approximately theequivalent of 1 kilowatt. Assuming slab speeds of 100 feet per minuteand 33.3 feet per minute, by way of examples: Power into slab:

(3 sec.) 953 =13,340 (linear) Assuming an efficiency of 50 percent, thepowers required are 39.6 megawatts and 15.6 megawatts.

Assuming use factors of 20 percent and 40 percent for the 100 ft./min.and 33.3 ft./min. speeds:

Particular reference is rnade now to FIG. 6, where a fourth embodimentof the invention is shown which is particularly suitable for use wheredifficulty is encountered, and melting the bottom surface of the bloomor slab results in loss of molten metal before it can be solidified. 1nthe embodiment of FIG. 6, the top and one side of the slab are treatedas the slab moves in one direction, for example, from right to left inthe figure. The slab thereafter passes into apparatus which may be ofconventional design and which is shown in block form at 141 for turningthe slab over, and the slab is thereafter passed again past theelectrodes where the other wide surface and the other side are treatedas the slab moves in the opposite direction, for example, from left toright in the figure. For the purposes of describing the embodiment ofFIG. 6, the top surface of the slab is designated 142, and the sidesurface first treated is designated 143. The shroud 145 has a pluralityof energized, not shown for convenience of illustration, disposedtherein, with pipes 146 and 147 for bringing an inert or reducing gas tothe arcing surface of the slab. It is seen that shroud 145 has two spraypipes or manifolds on the two sides thereof, these being shown at 148and 149, each of the spray pipes having a valve therein, the valve forpipe 149 being shown at 150. As the slab 140 is moving in the left-handdirection, the valve in pipe 148 is opened so that spray coming out ofthe spray pipe or manifold 148 cools successive portions of the slabsurface which have passed adjacent the electrodes within the shroud 145.On the other hand, after the slab has been turned over in apparatus 141,the valve in pipe 148 is turned off, and the valve 150 to manifold 149is opened so that spray pipe or manifold 149 sprays the upper oradjacent surface of the slab after said last-named surface passesadjacent the electrodes within the shroud. It will be understood thatthe lower housing 152 contains no electrodes in the embodiment of FIG.6. The lower housing 152 may if desired be replaced by rollers so thatan unbroken series of rollers supports the slab 140. As previouslystated some of the rollers are free to rotate, while other rollers aredriven, the driven rollers being disposed at spaced intervals along thelength of the slab, two driven rollers being designated and 171 withreversible drive motors 172 and 173 respectively.

The electrode or electrodes for treating the side 143 of the slab, asthe slab moves from a right to left direction are disposed within theshroud 155, which has spray pipes 156 and 157 disposed on the sidesthereof, respectively, it being understood that the spray pipes ormanifolds 156 and 157 have valves, not shown, individual thereto so thatsuccessive portions of the surface of the side of the slab can besprayed with a cooling fluid substantially immediately after they leavethe electrode area, depending upon which direction the slab is moving.It will further be understood that the housing 159 contains no electrodein the embodiment of FIG. 6. Means,

' not shown, is provided for bringing an inert gas into the shroud ofthe nonconsumable electrode constituting no part of the presentinvention, the annular arcing surface being shown at 200, the field coilat 201.

In FIG. 1, the upper and lower electrodes may be connected to oppositeterminals of a source of potential, the slab providing a neutral currentpath between the electrodes. In like manner, the side electrodes may beconnected to tenninals of opposite polarity of a source, the slab beingneutral but providing a current path between electrodes.

There have been provided then a number of embodiments of apparatussuitable for practicing the method of our invention, which is to passthe surfaces of a slab or bloom past an are or arcs where the surfacesare simultaneously or sequentially heated to a predetermined depth toremove defects therefrom, and the surfaces are promptly cooled after theheat treatment thereof by the arcs.

The term inert gas as employed in the claims appended hereto includesreducing gases and nonoxidizing gases.

The term slab" as used in the claims appended hereto includes blooms ofirregular shapes.

Whereas we have shown and described apparatus for practicingthe methodof our invention according to a number of embodiments thereof it will beunderstood that changes may be made and equivalents substituted withoutdeparting from the spirit and scope of the invention.

We claim:

1. A method of heat treating a slab of metal to remove defects therefromwhich comprises the steps of forming at least one electric are from anannular electrode extending to the slab in a path substantiallyperpendicular to a surface of the slab, utilizing a magnetic field tocause the are path to describe substantially annular repetitive rotativemovements about a substantially unchanged axis while maintaining the arcpath substantially perpendicular ro the surface of the slab, moving theslab in a substantially linear path at a predetermined speed, saidlinear path being substantially perpendicular to the arc path wherebysuccessive rotations of the are spot on the slab as the 'arc rotates aredisplaced from each other by an amount which is a function of the rateof arc movement and a function of the rate of slab movement, the ratesof movement I tially uniformly heated by the are spot directly thereon.

having an annular fluid-cooled arcing surface and with the arcingsurface of the electrode spaced from and substantially parallel to theadjacent flat surface of the slab, creating a potential differencebetween the electrode and the slab whereby an arc is formed between theslab and the arcing surface of the electrode, said are extending in adirection substantially parallel to the axis of the electrode andsubstantially perpendicular to said flat surface of the slab, generatinga magnetic field within the electrode with at least a strong componentextending substantially radially in a transverse direction across thearcirfg surface and exerting a force on the are which causes the arebetween electrode and slab surface to move in substantially repetitiveannular paths on the arcing surface and to take similar repetitive arcpaths on the slab surface as the slab moves, which last-named pathsextend a predetermined portion or all of the transverse distance acrossthe slab surface, the arc paths on the slab surface being displaced fromeach other a distance which'is a function of the speed of arc rotationand a function of the speed of slab movement, the rate of movement ofthe slab and the rate of movement of the are being selected with respectto each other whereby substantially every point on the slab surfacewithin said transverse distanceahas an arc spot thereto as the slabmoves in said linear path with a resulting increase in heatingefficiency and more uniform heating of the slab to a substantiallyconstant depth beneath the slab surface.

7. A rnethod for removing defects from a slab of metal havraga: leastthree substantially flat surfaces comprising the steps of moving theslab in a predetermined linear path at a predetermined speed, mountingat least three annular electrodes at predetermined positions along thepath of travel of the slab, each electrode having an annularfluid-cooled arcing surface and with the arcing surface of eachelectrode spaced from and substantially parallel to an adjacent anddifferent flat surface of the slab, creating potential differencesbetween all of the electrodes and the slab whereby arcs are formedbetween the slab and the arcing surfaces of all of the electrodes, eachare extending in a direction substantially parallel to the axis of theelectrode to which it takes place and substantially perpendicular to theflat slab surface to which it takes 2Q A method according to claim 1inclu'dirig'the additional step of quickly cooling successive portionsof said slab surface after said portions have been heated by theelectric arc.

3. A'method according to claim 1 wherein the substantially f annularrepetitive rotative movements of the arc describe an ellipse.

' passes substantially the entire surface.

5. A method according to claim 1 in which at least two arcs to theslabsurface are formed, the repetitive movement of one are being displacedfrom the repetitive movement of the other are laterally across the slabsurface, the rotations of the are spot of one are on the slab surfaceoverlapping the rotations of the are spot of the other are on the slabsurface as said slab moves in said linear path.

, 6. A method for removing defects from a slab of metal having at leastone substantially flat surface comprising the steps of moving the slabin a predetermined linear path at ,a predetermined speed, mounting anannular electrode at ,a predetermined position along the path of travelof the slab place, generating a magnetic field within each electrodewith at least a strong componentextending substantially radially in atransverse direction across the arcing surface of the electrode andexerting a force on the arc from the electrode which causes the arc tomove insubstantially repetitive annular paths on the arcing surface ofthe electrode and to take similar repetitive arc paths on the adjacentslab surface to which the arc takes place as the slab moves, eachrepetitive arc path on a slab surface extending a predetermined portionor all of the transverse distance across the last-named slab surface,the repetitive arc paths on'a' slab surface being displaced from eachother a distance which is a function of the speed of arc rotation and afunction of the speed of slab movement, the rates of movement of thearcs and the rate of movement of the slab being selected with respect toeach other whereby substantially every point on each slab surface withinthe transverse distance covered by the arc path on that surface has anare spot thereto as the slab moves in said linear path with resultingincreases in heating efficiency and more uniform heating of the slab tosubstantially constant depths beneath all of the slab surfaces.

8. The method of heat treating a substantially rectangular slab of metalto remove. defects therefrom which comprises the steps of moving theslab in a first direction along a predetermined path, mounting at leasttwo annular electrodes at predetermined positions along the path oftravel of the slab, each electrode having an annular fluid-cooled arcingsurface and with the arcing surface of each electrode spaced from andsubstantially-parallel to an adjacent and different flat surface of theslab, the two electrodes being so mounted with respect to each otherthat their arcing surfaces are substantially parallel respectively totwo slab surfaces which make substantially a angle with respect to eachother, creating potential differences between both of the two electrodesand the slab whereby arcs are formed between the slab and the arcingsurfaces of both of the electrodes, each arc extending in a directionsubstantially parallel to the axis of the electrode to which it takesplace and substantially perpendicular to the flat slab surface to whichit takes place, generating a magnetic field within each electrode withat least a strong component extending substantially radially in atransverse direction across the arcing surface of the electrode andexerting a force on the arc form the electrode which causes the arc tomove in substantially repetitive annular paths on the arcing surface ofthe electrode and to take similar repetitive arc paths on the adjacentslab surface to which the arc takes place as the slab moves, eachrepetitive arc path on a slab surface extending a predetermined portionor all of the transverse distance across the lastnamed slab surface, thearc paths on a slab surface being displaced from each other a distancewhich is a function of the speed of arc rotation and a function of thespeed of slab movement, the rates of movement of the arcs and the rateof movement of the slab being selected with respect to each otherwhereby substantially every point on the slab surfaces within thetransverse distances of arc movement has an are spot thereto as the slabmoves in said first direction with a resulting increase in heatingefficiency and more uniform heating of the slab to a substantiallyconstant depth beneath the slab surface, turning the slab over wherebythe two surfaces thereof not previously having arcs thereto are eachadjacent the arcing surface of one of the two electrodes, and causingthe slab to move in the opposite direction at a predetermined speedwhile maintaining arcs to said lastnamed two surfaces which move atpredetermined speeds selected in accordance with the last-named speed ofslab movement so that are spots occur on the two last-named slabsurfaces at substantially every point thereon within the lateraldistances of movement of the arcs.

9. The method according to claim 8 including the additional step ofquickly cooling the upper and lower surfaces and both the side surfacesafter the respective surfaces are heated by the arcs.

10. A method of heat treating a slab of metal toremove defects therefromcomprising the steps of forming an electric are from a fluid-cooledannular electrode arcing surface to at least one surface of the slab,generating a magnetic field which causes the arc to move substantiallycontinuously in repetitive generally annular paths over the arcingsurface, moving the slab along a predetermined path past the electrodewhile causing the arc to periodically traverse a recurring generallyannular path along a strip of the surface of the slab, the speed of arcmovement and the speed of slab movement being selected with respect toeach other whereby an are spot occurs at substantially every point onsaid surface within the dimensions of said strip with increased heatingefficiency and more uniformi ty in the depth to which the slab is heatedto a predetermined temperature, and quickly cooling successive portionsof the surface of the slab after said portions have been heated by theelectric arc.

11. A method of heat treating a multisurface slab of metal to removedefects therefrom which comprises the steps of forming an electric arefrom a fluid-cooled annular electrode to at least one surface of theslab, generating a magnetic field which causes the arc to movesubstantially continuously in generally repetitive paths over the arcingsurface, moving the slab along a predetermined path past the electrodewhereby the moving arc describes paths on the slab surface similar tothe paths on the arcing surface of the electrode, the speed of movementof the slab and the speed of movement of the are being selected withrespect to each other whereby substantially every point within thelateral dimension of the arc paths on the slab surface has an arc spotformed thereon with improved heating efficiency and more uniform heatingof the slab, providing an inert gas atmosphere at the instant portion ofthe slab surface being melted as the slab moves in said path, andquickly cooling successive portions of said slab surface after saidportions have been heated by the electric arc.

1. A method of heat treating a slab of metal to remove defects therefromwhich comprises the steps of forming at least one electric arc from anannular electrode extending to the slab in a path substantiallyperpendicular to a surface of the slab, utilizing a magnetic field tocause the arc path to describe substantially annular repetitive rotativemovements about a substantially unchanged axis while maintaining the arcpath substantially perpendicular ro the surface of the slab, moving theslab in a substantially linear path at a predetermined speed, saidlinear path being substantially perpendicular to the arc path wherebysuccessive rotations of the arc spot on the slab as the arc rotates aredisplaced from each other by an amount which is a function of the rateof arc movement and a function of the rate of slab movement, the ratesof movement being chosen with respect to each other whereby at least asubstantially rectangular portion of the slab surface is substantiallyuniformly heated by the arc spot directly thereon.
 2. A method accordingto claim 1 including the additional step of quickly cooling successiveportions of said slab surface after said portions have been heated bythe electric arc.
 3. A method according to claim 1 wherein thesubstantially annular repetitive rotative movements of the arc describean ellipse.
 4. A method according to claim 1 wherein the substantiallyannular repetitive rotative movements of the arc extend transverselyacross substantially the entire slab surface and in which saidrectangular portion of the slab surface encompasses substantially theentire surface.
 5. A method according to claim 1 in which at least twoarcs to the slab surface are formed, the repetitive movement of one arcbeing displaced from the repetitive movement of the other arc laterallyacross the slab surface, the rotations of the arc spot of one arc on theslab surface overlapping the rotations of the arc spot of the other arcon the slab surface as said slab moves in said linear path.
 6. A methodfor removing defects from a slab of metal having at least onesubstantially flat surface comprising the steps of moving the slab in apredetermined linear path at a predetermined speed, mounting an annularelectrode at a predetermined position along the path of travel of theslab having an annular fluid-cooled arcing surface and with the arcingsurface of the electrode spaced from and substantially parallel to theadjacent flat surface of the slab, creating a potential differencebetween the electrode and the slab whereby an arc is formed between theslab and the arcing surface of the electrode, said arc extending in adirection substantially parallel to the axis of the electrode andsubstantially perpendicular to said flat surface of the slab, generatinga magnetic field within the electrode with at least a strong componentextending substantially radially in a transverse direction across thearcing surface and exerting a force on the arc which causes the arcbetween electrode and slab surface to move in substantially repetitiveannular paths on the arcing surface and to take similar repetitive arcpaths on the slab surface as the slab moves, which last-named pathsextend a predetermined portion or all of the transverse distance acrossthe slab surface, the arc paths on the slab surface being displaced fromeach other a distance which is a function of the speed of arc rotationand a function of the speed of slab movement, the rate of movement ofthe slab and the rate of movement of the arc being selected with respectto each other whereby substantially every point on the slab surfacewithin said transverse distance has an arc spot thereto as the slabmoves in said linear path with a resulting increase in heatingefficiency and more uniform heating of the slab to a substaNtiallyconstant depth beneath the slab surface.
 7. A method for removingdefects from a slab of metal having at least three substantially flatsurfaces comprising the steps of moving the slab in a predeterminedlinear path at a predetermined speed, mounting at least three annularelectrodes at predetermined positions along the path of travel of theslab, each electrode having an annular fluid-cooled arcing surface andwith the arcing surface of each electrode spaced from and substantiallyparallel to an adjacent and different flat surface of the slab, creatingpotential differences between all of the electrodes and the slab wherebyarcs are formed between the slab and the arcing surfaces of all of theelectrodes, each arc extending in a direction substantially parallel tothe axis of the electrode to which it takes place and substantiallyperpendicular to the flat slab surface to which it takes place,generating a magnetic field within each electrode with at least a strongcomponent extending substantially radially in a transverse directionacross the arcing surface of the electrode and exerting a force on thearc from the electrode which causes the arc to move in substantiallyrepetitive annular paths on the arcing surface of the electrode and totake similar repetitive arc paths on the adjacent slab surface to whichthe arc takes place as the slab moves, each repetitive arc path on aslab surface extending a predetermined portion or all of the transversedistance across the last-named slab surface, the repetitive arc paths ona slab surface being displaced from each other a distance which is afunction of the speed of arc rotation and a function of the speed ofslab movement, the rates of movement of the arcs and the rate ofmovement of the slab being selected with respect to each other wherebysubstantially every point on each slab surface within the transversedistance covered by the arc path on that surface has an arc spot theretoas the slab moves in said linear path with resulting increases inheating efficiency and more uniform heating of the slab to substantiallyconstant depths beneath all of the slab surfaces.
 8. The method of heattreating a substantially rectangular slab of metal to remove defectstherefrom which comprises the steps of moving the slab in a firstdirection along a predetermined path, mounting at least two annularelectrodes at predetermined positions along the path of travel of theslab, each electrode having an annular fluid-cooled arcing surface andwith the arcing surface of each electrode spaced from and substantiallyparallel to an adjacent and different flat surface of the slab, the twoelectrodes being so mounted with respect to each other that their arcingsurfaces are substantially parallel respectively to two slab surfaceswhich make substantially a 90* angle with respect to each other,creating potential differences between both of the two electrodes andthe slab whereby arcs are formed between the slab and the arcingsurfaces of both of the electrodes, each arc extending in a directionsubstantially parallel to the axis of the electrode to which it takesplace and substantially perpendicular to the flat slab surface to whichit takes place, generating a magnetic field within each electrode withat least a strong component extending substantially radially in atransverse direction across the arcing surface of the electrode andexerting a force on the arc form the electrode which causes the arc tomove in substantially repetitive annular paths on the arcing surface ofthe electrode and to take similar repetitive arc paths on the adjacentslab surface to which the arc takes place as the slab moves, eachrepetitive arc path on a slab surface extending a predetermined portionor all of the transverse distance across the last-named slab surface,the arc paths on a slab surface being displaced from each other adistance which is a function of the speed of arc rotation and a functionof the speed of slab movement, the rates of movement of the arcs and therate of movement of the slab being selected with respect to each otherwhereby substantially every point on the slab surfaces within thetransverse distances of arc movement has an arc spot thereto as the slabmoves in said first direction with a resulting increase in heatingefficiency and more uniform heating of the slab to a substantiallyconstant depth beneath the slab surface, turning the slab over wherebythe two surfaces thereof not previously having arcs thereto are eachadjacent the arcing surface of one of the two electrodes, and causingthe slab to move in the opposite direction at a predetermined speedwhile maintaining arcs to said last-named two surfaces which move atpredetermined speeds selected in accordance with the last-named speed ofslab movement so that arc spots occur on the two last-named slabsurfaces at substantially every point thereon within the lateraldistances of movement of the arcs.
 9. The method according to claim 8including the additional step of quickly cooling the upper and lowersurfaces and both the side surfaces after the respective surfaces areheated by the arcs.
 10. A method of heat treating a slab of metal toremove defects therefrom comprising the steps of forming an electric arcfrom a fluid-cooled annular electrode arcing surface to at least onesurface of the slab, generating a magnetic field which causes the arc tomove substantially continuously in repetitive generally annular pathsover the arcing surface, moving the slab along a predetermined path pastthe electrode while causing the arc to periodically traverse a recurringgenerally annular path along a strip of the surface of the slab, thespeed of arc movement and the speed of slab movement being selected withrespect to each other whereby an arc spot occurs at substantially everypoint on said surface within the dimensions of said strip with increasedheating efficiency and more uniformity in the depth to which the slab isheated to a predetermined temperature, and quickly cooling successiveportions of the surface of the slab after said portions have been heatedby the electric arc.
 11. A method of heat treating a multisurface slabof metal to remove defects therefrom which comprises the steps offorming an electric arc from a fluid-cooled annular electrode to atleast one surface of the slab, generating a magnetic field which causesthe arc to move substantially continuously in generally repetitive pathsover the arcing surface, moving the slab along a predetermined path pastthe electrode whereby the moving arc describes paths on the slab surfacesimilar to the paths on the arcing surface of the electrode, the speedof movement of the slab and the speed of movement of the arc beingselected with respect to each other whereby substantially every pointwithin the lateral dimension of the arc paths on the slab surface has anarc spot formed thereon with improved heating efficiency and moreuniform heating of the slab, providing an inert gas atmosphere at theinstant portion of the slab surface being melted as the slab moves insaid path, and quickly cooling successive portions of said slab surfaceafter said portions have been heated by the electric arc.